This unnamed species of sapient humanoids need a life support system to operate in Earth's atmosphere. They are native to the rogue planet discovered entering the Sol System in 1951 and designated as Planet X by astronomers.
As it turns out, the aliens' homeworld was slowly dying out, leading them to develop an anti-gravitational system to allow it out of its native star system and into the solar system, and approaching Earth without disturbing its orbit, at a distance suitable for transportation of an invasion army. A single alien is sent first in a conventional spacecraft that lands in a small village in Scottland.
The invader comes armed with the ability (either natural or technological) to hypnotize humans and use them as laborers to construct a teletransport device, which would beam the invaders to Earth when Planet X makes the closest approach. The alien first makes contact with scientists Professor Elliot and Dr. Mears, the reporter Lawrence and the professor's daughter Enid. They act peacefully towards the alien, protect it from the suspicious villagers and even save its life when it needs help regulating its spacesuit; but fail in their attempts at communication. Using mathematical equations and music as a common ground, Dr. Mears secretly manages to communicate with the creature, but then violently attacks it and attempts to steal its formula for the material of which the spaceship's hull is made, which proves to be tougher than diamond. After escaping from the Professor's house, the alien seemingly becomes convinced that an aggressive colonization is needed and proceeds to kidnap and hypnotize locals (including Enid, Mears and the Professor) to work on the transport. Lawrence and the townspeople call for military help and manage to evacuate most of the hypnotized workers and kill the alien before the military destroy its ship and transport device. When the time comes, Planet X passes through Earth's orbit back to interstellar space to its next unfortunate destination.
Although the alien's actions are hard to interpret due to its inhuman nature, Enid later speculates that, hadn't Dr. Mears attacked the creature, things could have played out differently.